Thanks all...and hobo, thanks for the elaboration. To be honest, I wasn't sure if Economy to Economy Plus was considered an upgrade or not, and I didn't realize that you can't buy a double upgrade (seems odd...if you have the miles to spare and have spent the $$$ to reach that point, it seems that you should be able to purchase any upgrade combo you wish. Then again, from a marketing/ROI perspective, maybe not). I think you're absolutely right--Y to C is a more profound service upgrade than C to F.

This thread has been quite enlightening. I look forward to any further discussion.

I'll add another question to the mix...can you purchase an upgrade on, say, LH or LX using UA miles without status on any? Now that I think about it, my attempts to upgrade have always been on partner airlines, not on UA itself. This probably makes a big difference.